1. Field of the Invention
This invention relates to the production of N-acetyl-p-aminophenol by hydrogenation of p-nitrophenol to p-aminophenol, and concurrently acetylating the p-aminophenol with acetic anhydride.
2. Background
The conventional process for the production of N-acetyl-p-aminophenol involves reduction of p-nitrophenol to produce p-aminophenol which is then acetylated to produce N-acetyl-p-aminophenol. The reduction of p-nitrophenol to produce p-aminophenol involves hydrogenating the p-nitrophenol in the presence of catalysts such as aluminum. platinum, palladium, noble metal catallysts, oxides of platinum, palladium or noble metal catalysts, molybdenum sulfide, and platinum sulfide. Gaseous hydrogen is commonly used as a reducing agent. The acetylating agent is usually acetic anhydride. The reaction medium can be acetic acid water, a water-isopropanol mixture, or other inert medium.
This series of reactions has been carried on simultaneously as in U.S. Pat. No. 076.030 and U.S. Pat. No. 3,341,587. In both of these simultaneous reaction patents, p-nitrophenol and acetic anhydride are added at the beginning of the reaction sequence, and the hydrogenation of the p-nitrophenol takes place in the presence of acetic anhydride that is in excess over the amount of p-aminophenol in the system at any given moment. This reaction sequence has also been carried on in a step-wise manner in which the hydrogeantion of p-nitrophenol is completed prior to the acetylation step. This step-wise reaction scheme has been further refined in U.S. Pat. No. 4,264,525 which the hydrogenation step is interrupted and acetylation accompished followed by at least one further hydrogenation step each of which is followed by another acetylation step. all the while keeping the pH below about 7.0.
Because of its use as an analgesic, N-acetyl-p-aminophenol must be very pure and must not be colored. This invention produces N-acetyl-p-aminophenol that is less colored than that produced by prior art processes. Additionally, this invention produces N-acetyl-p-aminophenol in a shorter reaction time than prior art processes.